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We have combined observations of Galactic high-velocity HI from two surveys: a very sensitive survey from the Green Bank 140ft Telescope with limited sky coverage, and the less sensitive but complete Galactic All Sky Survey from the 64m Parkes Radio Telescope. The two surveys preferentially detect different forms of neutral gas due to their sensitivity. We adopt a machine learning approach to divide our data into two populations that separate across a range in column density: 1) a narrow line-width population typical of the majority of bright high velocity cloud components, and 2) a fainter, broad line-width population that aligns well with that of the population found in the Green Bank survey. We refer to these populations as dense and diffuse gas respectively, and find that diffuse gas is typically located at the edges and in the tails of high velocity clouds, surrounding dense components in the core. A fit to the average spectrum of each type of gas in the Galactic All Sky Survey data reveals the dense population to have a typical line width of ~20 km/s and brightness temperature of ~0.3 K, while the diffuse population has a typical line width of ~30 km/s and a brightness temperature of ~0.2 K. Our results confirm that most surveys of high velocity gas in the Milky Way halo are missing the majority of the ubiquitous diffuse gas, and that this gas is likely to contribute at least as much mass as the dense gas., Comment: 15 pages, 13 figures, accepted for publication in ApJ

Synchrotron emission pervades the Galactic plane at low radio frequencies, originating from cosmic ray electrons interacting with the Galactic magnetic field. Using a low-frequency radio telescope, the Murchison Widefield Array (MWA), we measure the free-free absorption of this Galactic synchrotron emission by intervening HII regions along the line of sight. These absorption measurements allow us to calculate the Galactic cosmic-ray electron emissivity behind and in front of 47 detected HII regions in the region $250^\circ < l < 355^\circ$, $|b| < 2^\circ$. We find that all average emissivities between the HII regions and the Galactic edge along the line of sight ($\epsilon_b$) are in the range of 0.24$\,\,\sim\,\,$0.70$\,\,$K$\,\,$pc$^{-1}$ with a mean of 0.40$\,\,$K$\,\,$pc$^{-1}$ and a variance of 0.10$\,\,$K$\,\,$pc$^{-1}$ at 88$\,\,$MHz. Our best model, the Two-circle model, divides the Galactic disk into three regions using two circles centring on the Galactic centre. It shows a high emissivity region near the Galactic centre, a low emissivity region near the Galactic edge, and a medium emissivity region between these two regions, contrary to the trend found by previous studies., Comment: 12 pages, 8 figures. Published on MNRAS. A conversion factor is corrected in this version. For the original published version, see https://academic.oup.com/mnras/article-abstract/465/3/3163/2544366/Galactic-synchrotron-emissivity-measurements?redirectedFrom=fulltext For its erratum, see https://academic.oup.com/mnras/article-abstract/472/1/828/4083643/Erratum-Galactic-synchrotron-emissivity

Measurement of the Galactic neutral atomic hydrogen (HI) column density, NHI, and brightness temperatures, Tb, is of high scientific value for a broad range of astrophysical disciplines. In the past two decades, one of the most-used legacy HI datasets has been the Leiden/Argentine/Bonn Survey (LAB). We release the HI 4$\pi$ survey (HI4PI), an all-sky database of Galactic HI, which supersedes the LAB survey. The HI4PI survey is based on data from the recently completed first coverage of the Effelsberg-Bonn HI Survey (EBHIS) and from the third revision of the Galactic All-Sky Survey (GASS). EBHIS and GASS share similar angular resolution and match well in sensitivity. Combined, they are ideally suited to be a successor to LAB. The new HI4PI survey outperforms the LAB in angular resolution (16.2', FWHM) and sensitivity (RMS: 43 mK). Moreover, it has full spatial sampling and thus overcomes a major drawback of LAB, which severely undersamples the sky. We publish all-sky column density maps of the neutral atomic hydrogen in the Milky Way, along with full spectroscopic data, in several map projections including HEALPix., Comment: 14 pages, 8 figures, A&A in press, corresponding authors: B. Winkel and P. M. W. Kalberla

We study the dynamic coarsening of wrinkles in an elastic sheet that is compressed while lying on a thin layer of viscous liquid. When the ends of the sheet are instantaneously brought together by a small distance, viscous resistance initially prevents the sheet from adopting a globally buckled shape. Instead, the sheet accommodates the compression by wrinkling. Previous scaling arguments suggested that a balance between the sheet's bending stiffness and viscous effects lead to a wrinkle wavelength $\lambda$ that increases with time $t$ according to $\lambda\propto t^{1/6}$. We show that taking proper account of the compression constraint leads to a logarithmic correction of this result, $\lambda\propto (t/\log t)^{1/6}$. This correction is significant over experimentally observable time spans, and leads us to reassess previously published experimental data., Comment: 12 pages. Version accepted in Phys. Rev. Fluids (with small correction to bibliography)

Context: The past decade has witnessed a large number of Galactic plane surveys at angular resolutions below 20". However, no comparable high-resolution survey exists at long radio wavelengths around 21cm in line and continuum emission. Methods: Employing the Very Large Array (VLA) in the C-array configuration and a large program, we observe the HI 21cm line, four OH lines, nineteen Halpha radio recombination lines as well as the continuum emission from 1 to 2GHz in full polarization over a large part of the first Galactic quadrant. Results: Covering Galactic longitudes from 14.5 to 67.4deg and latitudes between +-1.25deg, we image all of these lines and the continuum at ~20" resolution. These data allow us to study the various components of the interstellar medium (ISM): from the atomic phase, traced by the HI line, to the molecular phase, observed by the OH transitions, to the ionized medium, revealed by the cm continuum and the Halpha radio recombination lines. Furthermore, the polarized continuum emission enables magnetic field studies. In this overview paper, we discuss the survey outline and present the first data release as well as early results from the different datasets. We now release the first half of the survey; the second half will follow later after the ongoing data processing has been completed. The data in fits format (continuum images and line data cubes) can be accessed through the project web-page http://www.mpia.de/thor. Conclusions: The HI/OH/Recombination line survey of the Milky Way (THOR) opens a new window to the different parts of the ISM. It enables detailed studies of molecular cloud formation, conversion of atomic to molecular gas, and feedback from HII regions as well as the magnetic field in the Milky Way. It is highly complementary to other surveys of our Galaxy, and comparing different datasets allows us to address many open questions., Comment: Accepted for Astronomy & Astrophysics, 23 pages, 17 figures, a high-resolution version can be found at http://www.mpia.de/thor

Using atomic hydrogen (HI) data from the VLA Galactic Plane Survey we measure the HI terminal velocity as a function of longitude for the first quadrant of the Milky Way. We use these data, together with our previous work on the fourth Galactic quadrant, to produce a densely sampled, uniformly measured, rotation curve of the Northern and Southern Milky Way between $3~{\rm kpc} < R < 8~{\rm kpc}$. We determine a new joint rotation curve fit for the first and fourth quadrants, which is consistent with the fit we published in McClure-Griffiths \& Dickey (2007) and can be used for estimating kinematic distances interior to the solar circle. Structure in the rotation curves is now exquisitely well defined, showing significant velocity structure on lengths of $\sim 200$ pc, which is much greater than the spatial resolution of the rotation curve. Furthermore, the shape of the rotation curves for the first and fourth quadrants, even after subtraction of a circular rotation fit shows a surprising degree of correlation with a roughly sinusoidal pattern between $4.2 < R < 7$ kpc., Comment: 24 pages, 8 figures. Accepted for publication in the Astrophysical Journal

We describe the performance of the Boolardy Engineering Test Array (BETA), the prototype for the Australian Square Kilometre Array Pathfinder telescope ASKAP. BETA is the first aperture synthesis radio telescope to use phased array feed technology, giving it the ability to electronically form up to nine dual-polarization beams. We report the methods developed for forming and measuring the beams, and the adaptations that have been made to the traditional calibration and imaging procedures in order to allow BETA to function as a multi-beam aperture synthesis telescope. We describe the commissioning of the instrument and present details of BETA's performance: sensitivity, beam characteristics, polarimetric properties and image quality. We summarise the astronomical science that it has produced and draw lessons from operating BETA that will be relevant to the commissioning and operation of the final ASKAP telescope., Comment: Accepted for publication in PASA

We present a high-resolution study of five high-velocity clouds in the Magellanic Leading Arm region. This is a follow-up study of our widefield Parkes survey of the region in order to probe the multiphase structures of the clouds and to give an insight to their origin, evolution and distance. High-resolution data were obtained from the Australia Telescope Compact Array. By combining with single-dish data from the Galactic All-Sky Survey (GASS), we are able to probe compact and diffuse emission simultaneously. We identify resolved and unresolved clumps. Physical parameters were derived for both diffuse structure and compact clumps. The latter are cold with typical velocity linewidths of 5 km/s. We find a gradient in thermal halo pressure, hydrogen density and HI column density of HVC as a function of Galactic latitude. This is possibly the first observational evidence of varying distance in the Leading Arm region, with the leading part of the Leading Arm (LA II and III) probably being closer to the Galactic disc than the trailing end (LA I)., Comment: 17 pages, 11 figures, accepted for publication in MNRAS

There is evidence in 21cm HI emission for voids several kpc in size centered approximately on the Galactic centre, both above and below the Galactic plane. These appear to map the boundaries of the Galactic nuclear wind. An analysis of HI at the tangent points, where the distance to the gas can be estimated with reasonable accuracy, shows a sharp transition at Galactic radii $R\lesssim 2.4$ kpc from the extended neutral gas layer characteristic of much of the Galactic disk, to a thin Gaussian layer with FWHM $\sim 125$ pc. An anti-correlation between HI and $\gamma$-ray emission at latitudes $10^{\circ} \leq |b| \leq 20^{\circ}$ suggests that the boundary of the extended HI layer marks the walls of the Fermi Bubbles. With HI we are able to trace the edges of the voids from $|z| > 2$ kpc down to $z\approx0$, where they have a radius $\sim 2$ kpc. The extended HI layer likely results from star formation in the disk, which is limited largely to $R \gtrsim 3$ kpc, so the wind may be expanding into an area of relatively little HI. Because the HI kinematics can discriminate between gas in the Galactic center and foreground material, 21cm HI emission may be the best probe of the extent of the nuclear wind near the Galactic plane., Comment: 22 pages, 7 figures. To appear in ApJ

Filters that act by adsorbing contaminant onto their pore walls will experience a decrease in porosity over time, and may eventually block. As adsorption will generally be larger towards the entrance of a filter, where the concentration of contaminant particles is higher, these effects can also result in a spatially varying porosity. We investigate this dynamic process using an extension of homogenization theory that accounts for a macroscale variation in microstructure. We formulate and homogenize the coupled problems of flow through a filter with a near-periodic time-dependent microstructure, solute transport due to advection, diffusion, and filter adsorption, and filter structure evolution due to the adsorption of contaminant. We use the homogenized equations to investigate how the contaminant removal and filter lifespan depend on the initial porosity distribution for a unidirectional flow. We confirm a conjecture made in Dalwadi et al. (2015) that filters with an initially negative porosity gradient have a longer lifespan and remove more contaminant than filters with an initially constant porosity, or worse, an initially positive porosity gradient. Additionally, we determine which initial porosity distributions result in a filter that will block everywhere at once by exploiting an asymptotic reduction of the homogenized equations. We show that these filters remove more contaminant than other filters with the same initial average porosity, but that filters which block everywhere at once are limited by how large their initial average porosity can be., Comment: 23 pages with 7 figures. Submitted to the Journal of Fluid Mechanics

We investigate how observations of synchrotron intensity fluctuations can be used to probe the sonic and Alfv\'enic Mach numbers of interstellar turbulence, based on mock observations performed on simulations of magnetohydrodynamic turbulence. We find that the structure function slope, and a diagnostic of anisotropy that we call the integrated quadrupole ratio modulus, both depend on the Alfv\'enic Mach number. However, these statistics also depend on the orientation of the mean magnetic field in the synchrotron emitting region relative to our line of sight, and this creates a degeneracy that cannot be broken by observations of synchrotron intensity alone. We conclude that the polarization of synchrotron emission could be analyzed to break this degeneracy, and suggest that this will be possible with the Square Kilometre Array., Comment: 24 pages, 23 figures

We carried out a large program with the Karl G. Jansky Very Large Array (VLA): "THOR: The HI, OH, Recombination line survey of the Milky Way". We observed a significant portion of the Galactic plane in the first quadrant of the Milky Way in the 21cm HI line, 4 OH transitions, 19 radio recombination lines, and continuum from 1 to 2 GHz. In this paper we present a catalog of the continuum sources in the first half of the survey (l=14.0-37.9deg and l=47.1-51.2deg, |b|<1.1deg) at a spatial resolution of 10-25", with a spatially varying noise level of ~0.3-1 mJy/beam. The catalog contains ~4400 sources. Around 1200 of these are spatially resolved, and ~1000 are possible artifacts, given their low signal-to-noise ratios. Since the spatial distribution of the unresolved objects is evenly distributed and not confined to the Galactic plane, most of them are extragalactic. Thanks to the broad bandwidth of the observations from 1 to 2 GHz, we are able to determine a reliable spectral index for ~1800 sources. The spectral index distribution reveals a double-peaked profile with maxima at spectral indices of alpha = -1 and alpha = 0 , corresponding to steep declining and flat spectra, respectively. This allows us to distinguish between thermal and non-thermal emission, which can be used to determine the nature of each source. We examine the spectral index of ~300 known HII regions, for which we find thermal emission with spectral indices around alpha = 0. In contrast, supernova remnants (SNR) show non-thermal emission with alpha = -0.5 and extragalactic objects generally have a steeper spectral index of alpha = -1. Using the spectral index information of the THOR survey, we investigate potential SNR candidates. We classify the radiation of four SNR candidates as non-thermal, and for the first time, we provide strong evidence for the SNR origin of these candidates., Comment: 24 pages, 34 figures, accepted for publication in A&A

We present radio continuum and polarization images of the North Polar Spur (NPS) from the Global Magneto-Ionic Medium Survey (GMIMS) conducted with the Dominion Radio Astrophysical Observatory 26-m Telescope. We fit polarization angle versus wavelength squared over 2048 frequency channels from 1280 to 1750 MHz to obtain a Faraday Rotation Measure (RM) map of the NPS. Combining this RM map with a published Faraday depth map of the entire Galaxy in this direction, we derive the Faraday depth introduced by the NPS and the Galactic interstellar medium (ISM) in front of and behind the NPS. The Faraday depth contributed by the NPS is close to zero, indicating that the NPS is an emitting only feature. The Faraday depth caused by the ISM in front of the NPS is consistent with zero at b>50 degree, implying that this part of the NPS is local at a distance of approximately several hundred parsecs. The Faraday depth contributed by the ISM behind the NPS gradually increases with Galactic latitude up to b=44 degree, and decreases at higher Galactic latitudes. This implies that either the part of the NPS at b<44 degree is distant or the NPS is local but there is a sign change of the large-scale magnetic field. If the NPS is local, there is then no evidence for a large-scale anti-symmetry pattern in the Faraday depth of the Milky Way. The Faraday depth introduced by the ISM behind the NPS at latitudes b>50 degree can be explained by including a coherent vertical magnetic field., Comment: 9 pages, 8 figures, accepted for publication in ApJ. Some figures have been degraded to reduce sizes, for a high resolution version, see http://physics.usyd.edu.au/~xhsun/ms_nps.pdf

Filters whose porosity decreases with depth are often more efficient at removing solute from a fluid than filters with a uniform porosity. We investigate this phenomenon via an extension of homogenization theory that accounts for a macroscale variation in microstructure. In the first stage of the paper, we homogenize the problems of flow through a filter with a near-periodic microstructure and of solute transport due to advection, diffusion, and filter adsorption. In the second stage, we use the computationally efficient homogenized equations to investigate and quantify why porosity gradients can improve filter efficiency. We find that a porosity gradient has a much larger effect on the uniformity of adsorption than it does on the total adsorption. This allows us to understand how a decreasing porosity can lead to a greater filter efficiency, by lowering the risk of localized blocking while maintaining the rate of total contaminant removal., Comment: 21 pages with 6 figures

We present the first detection of HCO$^+$ absorption in the Magellanic System. Using the Australia Telescope Compact Array (ATCA), we observed 9 extragalactic radio continuum sources behind the Magellanic System and detected HCO$^+$ absorption towards one source located behind the leading edge of the Magellanic Bridge. The detection is located at LSR velocity of $v=214.0 \pm 0.4\rm\,km\,s^{-1}$, with a full width at half maximum of $\Delta v=4.5\pm 1.0\rm\,km\,s^{-1}$ and optical depth of $\tau(\rm HCO^+)=0.10\pm 0.02$. Although there is abundant neutral hydrogen (HI) surrounding the sightline in position-velocity space, at the exact location of the absorber the HI column density is low, $<10^{20}\rm\,cm^{-2}$, and there is little evidence for dust or CO emission from Planck observations. While the origin and survival of molecules in such a diffuse environment remains unclear, dynamical events such as HI flows and cloud collisions in this interacting system likely play an important role., Comment: Accepted for publication in ApJ. 6 pages, 2 figures, 2 tables

To study the atomic, molecular and ionized emission of Giant Molecular Clouds (GMCs), we have initiated a Large Program with the VLA: 'THOR - The HI, OH, Recombination Line survey of the Milky Way'. We map the 21cm HI line, 4 OH lines, 19 H_alpha recombination lines and the continuum from 1 to 2 GHz of a significant fraction of the Milky Way (l=15-67deg, |b|<1deg) at ~20" resolution. In this paper, we focus on the HI emission from the W43 star-formation complex. Classically, the HI 21cm line is treated as optically thin with column densities calculated under this assumption. This might give reasonable results for regions of low-mass star-formation, however, it is not sufficient to describe GMCs. We analyzed strong continuum sources to measure the optical depth, and thus correct the HI 21cm emission for optical depth effects and weak diffuse continuum emission. Hence, we are able to measure the HI mass of W43 more accurately and our analysis reveals a lower limit of M=6.6x10^6 M_sun, which is a factor of 2.4 larger than the mass estimated with the assumption of optically thin emission. The HI column densities are as high as N(HI)~150 M_sun/pc^2 ~ 1.9x10^22 cm^-2, which is an order of magnitude higher than for low mass star formation regions. This result challenges theoretical models that predict a threshold for the HI column density of ~10 M_sun/pc^2, at which the formation of molecular hydrogen should set in. By assuming an elliptical layered structure for W43, we estimate the particle density profiles. While at the cloud edge atomic and molecular hydrogen are well mixed, the center of the cloud is strongly dominated by molecular hydrogen. We do not identify a sharp transition between hydrogen in atomic and molecular form. Our results are an important characterization of the atomic to molecular hydrogen transition in an extreme environment and challenges current theoretical models.

We present polarimetric maser observations with the Australia Telescope Compact Array (ATCA) of excited-state hydroxyl (OH) masers. We observed 30 fields of OH masers in full Stokes polarization with the Compact Array Broadband Backend (CABB) at both the 6030 and 6035 MHz excited-state OH transitions, and the 6668-MHz methanol maser transition, detecting 70 sites of maser emission. Amongst the OH we found 112 Zeeman pairs, of which 18 exhibited candidate {\pi} components. This is the largest single full polarimetric study of multiple sites of star formation for these frequencies, and the rate of 16% {\pi} components clearly indicates the {\pi} component exists, and is comparable to the percentage recently found for ground-state transitions. This significant percentage of {\pi} components, with consistent proportions at both ground- and excited-state transitions, argues against Faraday rotation suppressing the {\pi} component emission. Our simultaneous observations of methanol found the expected low level of polarisation, with no circular detected, and linear only found at the less than or equal to 10% level for the brightest sources., Comment: 17 pages (72 pages including appendices), 9 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society

As we strive to understand how galaxies evolve it is crucial that we resolve physical processes and test emerging theories in nearby systems that we can observe in great detail. Our own Galaxy, the Milky Way, and the nearby Magellanic Clouds provide unique windows into the evolution of galaxies, each with its own metallicity and star formation rate. These laboratories allow us to study with more detail than anywhere else in the Universe how galaxies acquire fresh gas to fuel their continuing star formation, how they exchange gas with the surrounding intergalactic medium, and turn warm, diffuse gas into molecular clouds and ultimately stars. The $\lambda$21-cm line of atomic hydrogen (HI) is an excellent tracer of these physical processes. With the SKA we will finally have the combination of surface brightness sensitivity, point source sensitivity and angular resolution to transform our understanding of the evolution of gas in the Milky Way, all the way from the halo down to the formation of individual molecular clouds., Comment: 25 pages, from "Advancing Astrophysics with the Square Kilometre Array", to appear in Proceedings of Science

An attractive technique for forming and collecting aggregates of magnetic material at a liquid--air interface by an applied magnetic field gradient was recently addressed theoretically and experimentally [Soft Matter, (9) 2013, 8600-8608]: when the magnetic field is weak, the deflection of the liquid--air interface has a steady shape, while for sufficiently strong fields, the interface destabilizes and forms a jet that extracts magnetic material. Motivated by this work, we develop a numerical model for the closely related problem of solving two-phase Navier--Stokes equations coupled with the static Maxwell equations. We computationally model the forces generated by a magnetic field gradient produced by a permanent magnet and so determine the interfacial deflection of a magnetic fluid (a pure ferrofluid system) and the transition into a jet. We analyze the shape of the liquid--air interface during the deformation stage and the critical magnet distance for which the static interface transitions into a jet. We draw conclusions on the ability of our numerical model to predict the large interfacial deformation and the consequent jetting, free of any fitting parameter.